An approach to the simulation of control systems using the digital computer

Abstract A technique is described for the digital simulation of a wide range of open- and closed-loop control systems. The method allows the determination of the time domain performance (e.g. the step response) of a system containing any series combination of simple or quadratic leads, lags and pure integrations. A computer program has been developed which will produce the time domain performance of a system by teletyping into the computer the relevant constants of that system. The resulting response can be obtained in the form of a print out for permanent accurate record or as a graphical display on a cathode ray oscilloscope or on an XY recorder. The method is thus particularly useful in the interactive computer-aided design of control systems. The method has two principal advantages over conventional analogue computer methods. Firstly accuracy can be traded directly for computational time and thus initial tests in design optimisation can be made quickly, whilst final designs may be checked to an accuracy far greater than that obtainable on any analogue computer simply by using a longer computational time. The second advantage is that structural alterations can be made to the model of the system under test merely by teletyping the new data into the computer. Analogue computers must be repatched and potentiometers set with the attendant possibility of errors being made. The article contains a flow diagram and detailed listing of a FORTRAN program based on the technique described and adapted to the limited storage capacity (4k 12-bit words) of a PDP 8 computer with associated cathode ray display.